A communication cablexe2x80x94sometimes called an xe2x80x9cumbilical cordxe2x80x9dxe2x80x94allows for the reliable communication of data and the transfer of power between a base station and a remote apparatus. For example, such a cable allows for the transfer of data between a surface vessel and a manned submersible, and another such cable allows for the transfer of data between the submersible and a remote-controlled exploration robot. These same cables also respectively allow the surface vessel to provide electric power to the submersible, and the submersible to provide power to the robot.
Because a communication cable is often prone to twisting and tanglingxe2x80x94a cable that connects a surface vessel to a manned submersible can be more than a mile longxe2x80x94the cable is often formed from cable segments that are connected with rotary couplers. Each coupler serially connects two cable segments, and helps prevent twisting and tangling by allowing one segment to rotate freely with respect to the other segment. Moreover, for many of these applications it is required to deploy and retrieve the cable via a rotating stowage drum fixed to either the base or remote vessel.
And because a cable segment is typically formed from one or more bundles of filaments that each carry a different signal, a rotary coupler is designed to connect each filament from one cable segment to the same filament in the other segment. The filaments are typically electrically conductive wires, optical fibers, or a combination of both wires and fibers.
An electrical rotary couplerxe2x80x94one that interconnects cable segments that include only conductive wiresxe2x80x94is typically rugged enough for use in harsh environments such as water, is relatively inexpensive, and has a relatively high connection density (the number of wire connections per unit of cross-sectional area). Because an electrical signal can propagate between conductors that merely touch one another, an electrical rotary coupler typically includes a metal slip-ring assembly that maintains the respective electrical connections between the wires of the cable segments as one segment rotates with respect to the other segment. Because the slip-ring assembly is made out of metal, the electrical coupler is relatively rugged. That is, the coupler can withstand the jarring, pressure, and other effects that are often characteristic of harsh environments. Furthermore, because it has a simple design, the electrical coupler is relatively easy to manufacture, and is thus relatively inexpensive. And because adding slip ringsxe2x80x94typically one ring per cable wirexe2x80x94to the assembly increases the length, but not the width, of the coupler, the coupler""s connection density can be relatively high.
But unfortunately, an optical rotary couplerxe2x80x94one that interconnects cable segments that include at least some optical fibersxe2x80x94is typically more sensitive and expensive, and has a lower connection density, than an electrical rotary coupler. Optical signals cannot propagate between optical fibers merely because they touch. Therefore, an optical coupler typically includes a delicate and complex optical assembly that maintains the fibers in one cable segment in optical alignment with the corresponding fibers in the other cable segment as one cable segment rotates with respect to the other. Unfortunately, because the optical assembly is delicate, jarring, pressure, and other environmental effects may adversely affect it such that the fibers become misaligned. If this misalignment becomes to large, one must remove the coupler and recalibrate it, repair it, or replace it. Furthermore, because the optical assembly is complex, it is often difficult to manufacture, and thus is often expensive. In addition, because the complexity, and thus the cost, of the optical assembly often increase as the number of fibers increases, the coupler""s connection density and connection capacityxe2x80x94the total number of filaments that the optical coupler can interconnectxe2x80x94are often relatively low.
Referring to FIG. 1, a conventional dove prism 10 is typically derived from a lower portion of a conventional right-angle prism (not shown), and has sides 12 and 14, a ceiling 16, a base 18 which may or may not have a reflective coating, and ends 20 and 22 that are at equal angles, typically 45xc2x0, to the base 18. When an image 24 is incident to the end 20 as shown, the prism 10 projects an inverted mirror image 26 from the end 22. It is well known that as the prism 10 rotates through an angle xcex8 about a center axis 28, the projected image 26 rotates through an angle xe2x88x922xcex8 about the axis, or twice as far as the prism in the opposite direction. For example, if the prism 10 rotates 90xc2x0 in a counterclockwise direction, then the projected image 26 rotates 180xc2x0 in a clockwise direction. And if the prism 10 rotates 180xc2x0 such that the base 18 is at the top of the prism, the projected image 26 rotates a full 360xc2x0. Thus, for every full revolution of the prism 10, the projected image 26 rotates two full revolutions. Furthermore, it is well known that as the incident image 24 rotates through an angle xcex8 about the axis 28, the projected image 26 rotates through an angle xe2x88x92xcex8, or as far as the image 24 in the opposite direction.
Referring to FIGS. 2-5, the properties of the prism 10 of FIG. 1 are explained with reference to a reference plane 40 and a collimated light beam 42, which is incident to the end 20 of the prism, is projected from the end 22, and is parallel to the ceiling 16 and base 18 before it enters and after it exits the prism. The prism has a perpendicular height H between the ceiling 16 and base 18, and a length L along the length of the base 18. The prism 10 also has an index of refraction that allows the prism to have the characteristics described below.
FIGS. 2-4 illustrate how a 180xc2x0 revolution of the prism 10 about the axis 28 in one direction results in a 360xc2x0 revolution of the projected portion of the beam 42 about the same axis in the other direction.
FIG. 2 is a side view of the prism 10 in its 0xc2x0 position (the base 18 is coincident with the reference plane 40) and the light beam 42. The incident portion of the light beam 42 is a height Ha from the base 18, and the end 20 refracts the beam to a reflection point 44, which is a distance La from the end 20 and a distance Lb from the end 22. The end 22 refracts the reflected portion of the beam 42 such that the projected portion of the beam is a height Hb from the base 18.
FIG. 3 is a view of the prism 10 from the end 22, where the prism is in its 0xc2x0 position, the broken-line circle represents the incident portion of the beam 42, and the solid circle represents the projected portion of the beam. Assuming that the incident portion of the beam 42 is stationary, as the prism 10 rotates about the axis 28 in a clockwise direction, the projected portion of the beam rotates at about the axis in a counterclockwise direction at twice the rotational rate of the prism. The directions of these respective rotations are represented by the broken-line arrows. Conversely, as the prism 10 rotates about the axis 28 in a counterclockwise direction, the projected portion of the beam rotates about the axis in a clockwise direction at twice the rotational rate of the prism. The directions of these respective rotations are represented by the solid-line arrows.
FIG. 4 is a side view of the prism 10 in its 180xc2x0 position (the ceiling 16 is coincident with the reference plane 40) and parallel to the light beam 42. Because the projected portion of the beam 42 is in the same position with respect to the reference plane 40 as it was when the prism 10 was in its 0xc2x0 position, it is evident that the projected portion has undergone a full revolution about the axis 28 in response to the half revolution of the prism 10. Specifically, because the incident portion of the beam 42 has remained the height Ha above the reference plane 40, it is now a drop of Hb below the base 18. Consequently, using well-known geometrical principles, the end 20 refracts the beam 42 to a reflection point 46, which is the distance Lb from the end 20 and the distance La from the end 22. The end 22 refracts the reflected portion of the beam 42 such that the beam""s projected portion is a drop Ha below the base 18 and the height Hb above the reference plane 40.
FIG. 5 illustrates how rotation of the incident portion of the beam 42 about the axis 28 in one direction results in an equal rotation of the projected portion of the beam 42 about the axis in the other direction. FIG. 5 is a view of the prism 10 from the end 22, where the prism is in its 0xc2x0 position, the broken-line circle represents the incident portion of the beam 42, and the solid circle represents the projected portion of the beam. Assuming that the prism 10 is stationary, as the incident portion of the beam 42 rotates about the axis 28 in a clockwise direction, the projected portion of the beam rotates about the axis in a counterclockwise direction at the same rotational rate as the incident portion of the beam. These respective rotations are represented by the broken-line arrows. Conversely, as the incident portion of the beam 42 rotates about the axis 28 in a counterclockwise direction, the projected portion of the beam rotates about the axis in a clockwise direction at the same rotational rate as the incident portion of the beam. These respective rotations are represented by the solid-line arrows.
One embodiment of the invention is an optical coupler that includes a housing and includes first and second optical terminals and a prism disposed in the housing. One terminal is moveable with respect to the other, and the prism maintains an optical alignment between the terminals.
Because it includes a prism instead of a more complex and delicate optical assembly, such an optical coupler can often be less expensive and more rugged, and can often have a higher connection density, than prior optical couplers.